Correct the default styling of block elements.

Jonathan - I submit this PR just to make the point really, this clearly isn't an elegant way of doing it.  But I need the block display fixes that come from HTML5shiv, and I don't understand what HTMLDocument is, so I can't properly appreciate what your solution is trying to achieve.
FYI, This is for backwards compatibility, since the service previously had a polyfill called html5shiv.